Exhaust gas discharged from an internal combustion engine (for example, a diesel engine or a gasoline engine) may contain particulates such as soot.
Exhaust gas containing such particulates is purified by means of collecting the particulates through use of a filter. Therefore, if the filter suffers breakage or a like failure, unpurified exhaust gas is discharged directly to the downstream side of the filter.
Therefore, there has been demanded a particulate detection system which can detect the quantity of particulates contained in exhaust gas in order to directly measure the quantity of particulates contained in exhaust gas or to detect a failure of the filter.
For example, Patent Document 1 discloses a particulate measurement method and apparatus. In the method disclosed in Patent Document 1, clean gas is ionized by means of corona discharge, the ionized gas containing positive ions is mixed with exhaust gas which is introduced from an exhaust pipe into a channel and which contains particulates, so as to electrify the particulates, and the particulates are then released to the exhaust pipe. A current (signal current) which flows in accordance with the quantity of the released, electrified particulates is detected so as to detect the particulate concentration.
Also, Patent Document 2 discloses a concrete structure of a particulate sensor which uses such ions generated by means of corona discharge. As disclosed in Patent Document 2, a constant current circuit is generally used as a power supply circuit for supplying electrical power for corona discharge, and when corona discharge is to be produced, constant current of, for example, about 5 μA is supplied to an electrode for corona discharge.